Story highlights

They have extracted millions of dollars in ransom from small businesses

So-called Cryptolocker has been used to attack about 200,000 computers

Two tech companies are offering a free tool to defeat sophisticated malicious software that criminals have used to extract millions of dollars in ransom payments from small businesses and other computer owners.

The software from FireEye and Fox IT can help victims decrypt files that were locked by the malware Cryptolocker.

The scheme typically worked this way: Victims would click a seemingly legitimate email, which automatically downloaded the malware on their computers.

Upon infection, victims would lose access to their files and criminals would demand payment in order to regain access.

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The malware remains defunct, but there are many victims whose files were locked by criminals and remain held hostage.

Since emerging in 2013, Cryptolocker has been used to attack about 200,000 computers, half of which were in the United States. In its first two months, criminals extorted an estimated $27 million from victims.

David Hickton, the U.S. attorney in Pittsburgh whose office led the investigation, said in June that Cryptolocker victims included the police department in Swansea, Massachusetts.

It paid a ransom to cyber criminals to restore access to its files after its systems were infected.